This invention relates to circuit interrupters.
A typical circuit interrupter comprises a pair of separable contact structures, at least one of which is movable, for interrupting an electric current flowing therethrough, and an operating mechanism operatively connected to at least one of the separable contact structures for opening and closing the contacts in response to overcurrent conditions in the circuit. The movable contact structure generally includes a pivotal arm support operated by the operating mechanism, a movable contact arm pivotally supported on the arm support and a biasing spring disposed between the arm support and the movable contact arm for biasing the movable contact arm toward a predetermined position relative to the arm support. The movable contact arm extends in a structurally parallel and an electrically opposite direction (with respect to current flow) to the stationary contact structure in order that an electromagnetic repulsive force, that separates the separable contacts against the biasing action of the spring, may be generated between the movable and the stationary contact structures during an overcurrent condition.
One example of the circuit interrupter of the above-described type is disclosed in U.S. Pat. No. 3,525,959 issued to Ellsworth et al. In this circuit interrupter, a movable contact structure comprises a switch arm secured to and projecting perpendicularly from a cross bar which rotatably extends through three pole units of the circuit interrupter, and a movable contact arm carrying a movable contact element at its tip portion and which is pivoted on a projecting end of the switch arm at its end portion so as to constitute, together with the switch arm, an elongated movable contact structure pivotal about a pivot axis located substantially at a midway portion of the entire movable contact structure. When the movable contact structure is to be actuated by an operating mechanism due to an overcurrent flowing through the circuit interrupter, the switch arm is rotated to separate the contacts about its pivot axis which is also the pivot axis of the cross bar. However, when an abnormally high overcurrent flows, this high current must be immediately interrupted and a mechanism for this immediate interruption is known wherein an electromagnetic repulsive force generated by an electric current flowing in opposite directions in a pair of parallel conductors, is utilized. With the conventional design utilizing the electromagnetic repulsive force, however, the distance from the movable contact element to the pivot axis of the movable contact arm is much shorter than that from the movable contact element to the pivot axis of the overall movable contact structure, which is also the pivot axis of the cross bar. Therefore, the movable contact arm must be rotated by a larger angle for obtaining the same separation distance, and the spring between the movable contact arm and the switch arm requires a larger deflection, resulting in a slow response speed and larger spring fatigue.
The circuit interrupter disclosed in the above cited patent utilizes in the movable contact structure a torsion spring disposed between the movable contact arm and the switch arm for providing the required contacting pressure between the movable and the stationary contact elements. This spring, which is relatively strong, must be assembled into the movable contact structure with substantial difficulty because of its strong spring force.
Also, with the conventional circit interrupter disclosed in the above cited patent, when the movable contact structure is moved to a separated, open position, movement of the structure is limited by a metallic stop mounted on the inner wall of the housing at the position above the movable contact arm which the contact arm abuts at its top surface or back surface opposite to the contact element. However, repeated strong impacts due to the abutment of the movable contact arm against the stop often causes deformations or distortions in the movable contact arm, resulting in a poor electrical contact between the movable and the stationary contact elements when they are closed.
Also, since the movable contact arm carrying the contact element thereon is pivotally connected at the tip of the switch arm, it has been very difficult to keep the alignment between the longitudinal axes of the movable contact arm and the switch arm, resulting in the movable contact element not being in registry with the stationary contact element even in the closed position.